This invention is generally in the area of Fischer-Tropsch synthesis.
The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Although natural gas includes some nitrogen- and sulfur-containing compounds, methane can be readily isolated in relatively pure form from natural gas using known techniques. Many processes have been developed which can produce fuel compositions from methane. Most of these process involve the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d).
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes a broad spectrum of products, ranging from methane to wax, which includes a significant amount of hydrocarbons in the distillate fuel range (C5-20).
Methane tends to be produced when chain growth probabilities are low, which is generally not preferred. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. The wax can be processed to form lower molecular weight products, but this processing often results in undesired formation of C1-4 products. Paraffinic Fischer-Tropsch products tend to be mostly linear, and tend to have relatively low octane values, relatively high pour points and relatively low sulfur contents. They are often isomerized to provide products with desired boiling ranges and pour point values.
Many isomerization catalysts require low levels of sulfur and nitrogen impurities, and feedstreams for these catalysts are often hydrotreated to remove any sulfur and nitrogen compounds. When isomerization processes are carried out with non-sulfided catalysts, various side reactions, such as hydrogenolysis (hydrocracking), can occur, producing undesired C1-C4 hydrocarbons. One approach to dealing with this limitation is to suppress hydrogenolysis by incorporating a small amount of sulfur-containing compounds into the feed, or by using other hydrocracking suppressants. A disadvantage of this approach is that it adds sulfur compounds to an otherwise essentially sulfur-free composition, which may not be desired.
It would be advantageous to provide additional processes for treating Fischer-Tropsch products which maximize formation of a mid-distillate (C5-20) product stream. The present invention provides such processes.
An integrated process for producing a hydrocarbon stream, preferably including predominantly C5-20 normal and iso-paraffins, is disclosed. The process involves isolating a methane-rich stream, i.e. predominantly a C4xe2x88x92 stream, and a C5+ stream (xe2x80x9cnatural gas condensatexe2x80x9d) from a natural gas source. The methane-rich stream is converted to syngas, and the syngas used in a hydrocarbon synthesis process, for example, Fischer-Tropsch synthesis.
In a first embodiment, one or more fractions from the hydrocarbon synthesis are blended with one or more crude oil derived fractions, and, optionally, the natural gas condensate, such that the overall sulfur content of the blend is less than about 200 ppm. If necessary, the crude oil fractions and/or natural gas condensate can be treated to lower the sulfur content so that the blend has an acceptable sulfur level. The fraction from the hydrocarbon synthesis may include, for example, C5-20 hydrocarbons, C20+ hydrocarbons, or C5+ hydrocarbons.
The blended hydrocarbons are subjected to hydroprocessing conditions. Olefins and oxygenates are hydrotreated to form paraffins. Paraffins are subjected to hydroisomerization conditions to form isoparaffins. Hydrocarbons with chain lengths above a desired value, for example, C24, are hydrocracked.
In this embodiment, the hydroprocessing catalysts are noble metal-containing catalysts, which tend to be sensitive to sulfur concentrations above about 200 ppm. The catalysts preferably have high selectivity for C5+ products, such that a significant C4xe2x88x92 fraction is not formed. Because the catalysts minimize the hydrogenolysis that would otherwise form C1-4 hydrocarbons, C20+ products can be combined with the natural gas condensate and the hydroprocessing conditions can be adjusted, for example, to maximize formation of a C5-20 hydrocarbon product in the distillate fuel range, or formation of a C20+ fraction in the lube base oil range, with mid-distillate products having carbon numbers predominately in the C5-20 range being particularly preferred.
In a second embodiment, one or more fractions from the hydrocarbon synthesis are blended with one or more crude oil derived fractions, and, optionally, the natural gas condensate, as in the first embodiment, but wherein the overall sulfur content of the blend is more than about 200 ppm. The blended hydrocarbons are subjected to hydroprocessing conditions as in the first embodiment, but using hydroprocessing catalysts that are not sulfur-sensitive. Preferably, at least one of the catalyst components is a pre-sulfided catalyst, for example, a pre-sulfided Group VM non-noble metal or a Group VI metal (e.g. tungsten or molybdenum) catalyst. The sulfur compounds present in the crude oil fractions and/or natural gas condensate can act as a hydrocracking suppressant, and minimize the amount of hydrocracking (hydrogenolysis) which would otherwise occur during the hydroprocessing reaction and form undesired C4xe2x88x92 products.
After the hydroprocessing steps, any remaining heteroatom-containing compounds can be removed, for example, using adsorption, extractive Merox or other means well known to those of skill in the art.
Optionally, at least a portion of the C2-4 products from the hydrocarbon synthesis step can be subjected to further processing steps, for example, olefin oligomerization, to provide an additional C5-20 product stream. This product stream may also be hydroprocessed in combination with the crude oil fractions, hydrocarbon synthesis products and/or natural gas condensate.